The
default background color for the Composition panel is black, but
you can change the color at any time.
Choose Composition > Background Color,
and then click the color swatch or use the eyedropper to pick a
color.
Note: When you add one composition to another (nesting),
the containing composition’s background color is preserved, and
the nested composition’s background becomes transparent. To preserve
the nested composition’s background color, create a solid-color
layer to use as a background layer in the nested composition.